Page 254 - PPL-engelsk 2025

Meteorology

The diagram shows a geostrophic wind (red

arrow) between two isobars.

The wind will have a strength of 20 knots at 2000

feet and will move parallel to the isobars.

3.4.3.2 Surface wind

Surface wind (wind in the friction layer) is less affected by the Coriolis force

because wind speed is reduced by friction.

This means that the wind

direction in surface winds

will be from a lower

direction, in degrees, than

the geostrophic wind.

Surface winds will:

- In high pressure, move to the right, diagonally away from the high pressure

- In low pressure, move to the left, diagonally towards the low pressure

Friction causes wind strength to gradually decrease from about 2000 feet and

lower.

The diagram here shows a surface wind (red

arrow) with a deviation of 20-30 degrees relative

to the isobars.

At the Earth's surface, the wind will have a speed

of 10 knots.

Over ocean areas, the difference between the geostrophic wind and surface

wind is not as large. The wind over water will be about 70% to 85% of the

geostrophic wind's strength. The difference in wind direction will be about 10

degrees (over land, it's 20-30 degrees).

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 254